This part of the educational course covers MRI techniques used to image the short/ultra-short T2 spins, abundant for example in the cartilage-bone interface and discuss the potential of the methods in the assessment of the short T2 species. The lectures will cover the main imaging methods, namely the UTE, ZTE, and SWIFT imaging sequences and their applications in the musculoskeletal system, especially in cartilage, bone, and the interface in between.
Target Audience
Trainees, researchers, and clinicians who are interested in short/ultra-short echo time imaging methods for short T2 species, especially in the musculoskeletal system.Educational Objectives
By attending this part of the educational course, participants should be able to explain the MRI techniques used to image the short/ultra-short T2 spins and discuss their potential in the assessment of the short T2 species. The participants can also identify potential pitfalls related to the different short T2 imaging methods.Purpose
Changes occur to both the deep cartilage and subchondral bone in musculoskeletal diseases such as osteoarthritis, and interactions between these adjacent tissues are important. However, due to the extremely short T2/T2* relaxation times of the tissues involved, advanced MRI techniques with sensitivity to ultra-short T2 relaxation times are required to obtain signal. Interactions between the tissues as well as the changes in the interface are relevant to the progression of OA, and if properly monitored, could provide insight in to the degenerative status of the joint compartments and the tissues.Methods and results
By the establishment of ultra-short echo time imaging methods, imaging of tissues with extremely rapid signal decay, i.e. ultra-short T2/T2* relaxation times has become feasible (1). The methods for imaging short T2 species are generally based on FID projection imaging, i.e. not using standard gradient echo or spin echo sequences that have echo times in the range of few to several milliseconds, but instead rely on excitation and immediate acquisition of an FID in the presence of a gradient inducing spatial encoding (1-5), resulting in ultra-short (microsecond-range) acquisition delay. Such methods are collectively known as ultra-short echo time sequences, examples of which are the zero echo time (ZTE) (6), ultra-short echo time (UTE) (7) and Sweep Imaging with Fourier Transform (SWIFT) (3,8-11) sequences.
For imaging the deep layers of articular cartilage, the calcified cartilage and bone-cartilage interface, these sequences offer obvious benefits by allowing capturing signals from the shortest T2 spins. An interesting bright signal feature, completely void in conventional sequences, has been observed at the bone-cartilage interface in ultra-short echo time imaging (12-14). This bright feature has been suggested to arise from the calcified cartilage and the deep layers of articular cartilage. Furthermore, it was shown that in a case of osteoarthritic specimen, the pattern at the interface demonstrated changes (13). It is highly likely that this feature observed only with ultra-short echo time imaging methods provides important insight into the degenerative status of the joint.
Besides plain imaging, several quantitative ultra-short echo time imaging methods have been introduced for the assessment of various musculoskeletal tissues, including T2/T2* mapping (15-18), T1rho mapping (19-21), adiabatic T1rho mapping (10,22) as well as T1 mapping (8,23) and magnetization transfer (24,25). For quantitative imaging, the same benefit of acquiring signal from the fastest relaxing spins is possible, enabling even quantitation of cortical bone.
While the ultra-short echo time imaging methods offer numerous benefits with their sensitivity to fast relaxing spins, they also have their specific downsides. Perhaps the first and foremost is the requirement for the scanner hardware to be capable of extremely rapid switching between transmit and receive, as required by these methods. For pre-clinical scanners, this is usually not a problem, but for clinical scanners, hardware upgrades may be required to reach the necessary switching times. Secondly, being projection imaging methods, these methods tend to be almost exclusively non-cartesian, thereby requiring more complex image reconstruction methods than the fast Fourier transform, making the image reconstruction more challenging. Depending on the specific ultra-short echo time method, there can be limitations in the achievable flip angle or excitation bandwidth, or in the sampling of the center of the k-space, which may be insufficient. ZTE and SWIFT sequences are also inherently 3-D sequences and thus require acquiring 3-D volumes thereby possibly excluding cases where a rapid 2-D scan might be desirable. Implementations on the clinical platform may not be available but are becoming more and more accessible and this trend is expected to continue. (1-6)
Discussion
Ultra-short echo time imaging methods are becoming more and more popular and provide possibilities in imaging tissues with extremely rapid T2/T2* relaxation. These methods find excellent use in the musculoskeletal applications, where most of the tissues of interest, such as the bone-cartilage interface, menisci, tendons, ligaments, entheses and bone exhibit abundance of short T2 species.1. Garwood M. MRI of fast-relaxing spins. J Magn Reson 2013;229:49-54. doi: 10.1016/j.jmr.2013.01.011.
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